The nod for the Tea Party backed challenger comes days after McConnell struck a deal with Senate Democratic Leader Harry Reid to reopen the government and avoid default.

"Matt Bevin is a true conservative who will fight to stop the massive spending, bailouts, and debt that are destroying our country. He is not afraid to stand up to the establishment and he will do what it takes to stop Obamacare," says Matt Hoskins, executive director of SCF.

Founded by former Sen. Jim DeMint of South Carolina, the group has attacked McConnell's leadership over the past several weeks in the ongoing GOP civil war. It had been expected the group would back Bevin for some time.

McConnell's campaign hasn't taken those criticisms lying down, however. Last month, the senator's re-election team said no group is more responsible for a Democratic majority in the Senate than SCF and have described the group as a "self-serving" organization.

This continues infighting in the GOP, which grew to new heights during the government shutdown.

Just this Thursday Sen. Orin Hatch of Utah blasted conservative activist groups and warned the political arm of Heritage Foundation, which is now headed by DeMint, that it could lose support from donors for attacking Republican incumbents.

"We know that winning this primary won't be easy. Mitch McConnell has the support of the entire Washington establishment and he will do anything to hold on to power. But if people in Kentucky and all across the country rise up and demand something better, we're confident Matt Bevin can win this race," says Hoskins.

McConnell aides are dismissing the endorsement mostly, and point to a poor fundraising total by Bevin's campaign reported in the third quarter with just above $820,000 that mostly came from the businessman's own pocket.

SCF joins The Madison Project and The United Kentucky Tea Party in endorsing Bevin.

"Matt Bevin now has the dubious honor of standing with a self-serving D.C. fundraising group that made its name by recruiting and promoting unelectable candidates that ensured Barack Obama a majority in the Senate," she says. "They clearly care less about Kentuckians than they do about their reputation for supporting laughably bad candidates. Now they can add a New England bailout recipient who claims he went to MIT to their roster of notable failures."